1. Field of the Invention
The present invention relates to a pressure sensor using a surface acoustic wave element.
2. Description of Related Art
JP-A-61-80024 discloses a pressure sensor using a surface acoustic wave (SAW) element. A substrate to generate surface acoustic wave has a diaphragm part, and a comb-teeth electrode is arranged on the diaphragm part as a resonator. When a pressure is applied to the diaphragm part, a surface stress of the diaphragm part is varied, such that an acoustic velocity is varied. Further, a variation of an electrode interval of the electrode changes a resonation frequency of the resonator. The applied pressure can be detected by the change of the resonation frequency.
JP-A-2008-185460 or JP-A-2007-114094 discloses a pressure sensor using a strain gauge sensor chip without the SAW element. A pressure sensor disclosed in JP-A-2008-185460 has a sensor chip mounted to a diaphragm part to receive and detect pressure. A pressure sensor disclosed in JP-A-2007-114094 has a diaphragm part, a pressure transmitting part and a strain part. Pressure received by the diaphragm part is transmitted to the strain part through the pressure transmitting part, and a sensor chip is mounted to the strain part, not to the diaphragm part.
The strain gauge sensor chip of the pressure sensor disclosed in JP-A-2008-185460 or JP-A-2007-114094 is changed to a SAW element sensor chip so as to provide a prototype pressure sensor.
The prototype pressure sensor is shown in FIGS. 28 and 29.
A pressure sensor J1 shown in FIG. 29 includes a diaphragm part J2, a sensor chip J3 and an adhesion layer J4. The sensor chip J3 is mounted to a top face of the diaphragm part J2 opposite from a pressure-receiving face. All back face area J4a of the sensor chip J3 shown in a hatched area of FIG. 28 is fixed to the diaphragm part J2 through the adhesion layer J4. The area J4a corresponds to an area on which the adhesion layer J4 is arranged.
The sensor chip J3 has a SAW element defined by a substrate J5 and a comb-teeth electrode J6 arranged on the substrate J5, so as to generate surface acoustic wave. The substrate J5 is made of a 128° Y-cut X-direction-propagating lithium niobate substrate.
The comb-teeth electrode J6 is arranged on the sensor chip J3 in a manner that a resonator is defined by the SAW element. A pressure is detected by a variation of a resonation frequency, similarly to the pressure sensor disclosed in JP-A-61-80024.
Specifically, when the diaphragm part J2 receives pressure in an arrow direction of FIG. 29, stress is generated to a top face of the diaphragm part J2 in a radial direction and a circumferential direction. Stress distribution is formed to be symmetric with respect to a center point of the diaphragm part J2.
At this time, because all the back face of the sensor chip J3 is bonded to the diaphragm part J2, the sensor chip J3 is restrained to the diaphragm part J2 uniformly in all the direction. Therefore, the same stress is applied to the sensor chip J3 as the diaphragm part J2.
As shown in arrow directions of FIG. 28, the stress is resolved into a SAW transmitting direction stress P1 defined in a direction of transmitting surface acoustic wave, and a perpendicular direction stress P2 defined in a direction perpendicular to the transmitting direction. A resonation frequency f is defined by dividing an acoustic velocity v by a resonation period 2L (f=v/2L), in which L represents an electrode interval.
Therefore, when the diaphragm part J2 receives pressure, a variation Δf of the resonation frequency of the sensor chip J3 is a sum of a variation ΔL of the electrode interval L, a variation ΔV1 of the acoustic velocity due to the SAW transmitting direction stress P1, and a variation ΔV2 of the acoustic velocity due to the perpendicular direction stress P2 (Δf/f=ΔL/L+ΔV1/V+ΔV2/V), in which V represents an acoustic velocity of surface acoustic wave corresponding to a transmission speed.
However, a direction of the variation ΔV2 is opposite from directions of the variations ΔV1, ΔL. Therefore, pressure detecting sensitivity of the sensor chip J3 may be decreased, because the variation ΔV2 cancels the variations ΔV1, ΔL.
Specifically, when the diaphragm part J2 has a predetermined thickness, the SAW transmitting direction stress P1 is a tensile stress in the transmitting direction, and the perpendicular direction stress P2 is a tensile stress in the perpendicular direction, in all area of the diaphragm part J2.
Therefore, due to the tensile stress P1 in the transmitting direction, the electrode interval L is increased, such that the resonation frequency is lowered. Further, due to the tensile stress P1 in the transmitting direction, the acoustic velocity is lowered in the transmitting direction, such that the resonation frequency is further lowered.
In contrast, due to the tensile stress P2 in the perpendicular direction, the acoustic velocity is raised in the transmitting direction, such that the resonation frequency is raised. Therefore, stress detecting sensitivity may be decreased, because variations of the resonation frequency are canceled by each other.
As shown in FIG. 28, the electrode J6 is located at an approximately center position of the diaphragm part J2. Even when a position or direction of the electrode J6 is changed, the pressure detecting sensitivity is decreased.
When the SAW element is used as the resonator, pressure is detected by the variation of the resonation frequency. In contrast, when the SAW element is used as a filter element, pressure is detected by a variation of a delay time.
Similar disadvantage will be generated, if the SAW element is used as the filter element, because the variation of the acoustic velocity due the SAW transmitting direction stress P1 and the variation of the acoustic velocity due the perpendicular direction stress P2 have directions opposite from each other.
Similar disadvantage will be generated, if the substrate J5 is made of other substrate different from the 128° Y-cut X-direction-propagating lithium niobate substrate.
The above disadvantage is generated when all the face of the sensor chip J3 is bonded to the diaphragm part J2. Similarly, the above disadvantage will be generated in the pressure sensor disclosed in JP-A-61-80024. When the diaphragm part of the substrate receives pressure, the perpendicular direction stress P2 is applied to the diaphragm part, because stress is applied to the diaphragm part in a radial direction.